Processes which allow dissimilar polymers to be combined in multilayered products have been developed commercially to manufacture films, sheet, pipe and tubing, and containers. In these processes, polyolefin resins such as polyethylene or polypropylene are commonly used and co-laminated with polyamides, ethylene-vinyl alcohol, vinylidene chloride resins, or other polymers, often by using a coextrusion process. The polyolefin layers in these products normally comprise the major fraction of the total thickness to provide strength and toughness at relatively low cost. The other resins used in thinner layers generally provide special barrier properties such as resistance to the passage of oxygen and other gases.
Since most polymers which are dissimilar to polyolefins do not adhere to polyolefins when colaminated, it has been necessary to develop special coextrudable adhesive resins to bond the polyolefin layer to the dissimilar polymer layer. Thus, various modifications have been made in the past to ethylene polymers and copolymers to provide adhesive resins suitable for use with polyamides or vinylidene chloride resins.
In general, polymers are dissimilar and are incompatible or have limited compatibility when they have different compositional contents, since long chain molecules greatly reduce the entropy of mixing and the heat of mixing is most often small and positive. Under these conditions, there is little or no diffusion of molecules across the interface nor strong adsorption of molecules from one polymer phase onto the other. The presence of these factors plays a crucial role during the formation of an adhesive bond and the subsequent strength of the adhesive bond when subjected to triaxial stresses imposed or caused by the differential expansion or contraction of the two bonded materials.
One class of polymers desirable for combining with polyolefins via multiple-layer co-lamination are the melt processable fluoropolymers, examples of which are ethylene-chlorotrifluoroethylene copolymers (E-CTFE) available under the trademark Halar.RTM., vinylidene fluoride polymers, chlorotrifluoroethylene copolymerized with vinylidene fluoride available under the trademark Aclon.RTM., tetrafluoroethylene, hexafluoropropylene, and copolymers thereof, and so on. These polymers are characterized by high chemical inertness to attack by strong chemicals such as concentrated mineral acids, oxidizing chemicals, and other highly reactive substances. In combination with polyolefins, these fluoropolymer layers would provide new types of containers, sheet products, and tubular shapes for use in contact with strong chemicals. For example, bottles may be made from high density polyethylene resins (HDPE) having a thin layer of E-CTFE on the inside surface to contain strong acids, or polypropylene pipe with a thin E-CTFE layer to convey these acids.
In light of the above desirable applications there has long been a need for a satisfactory, preferably coextrudable, adhesive resin for bonding polymers with little or no compatibility.